The present invention relates to a light control sheet wherein a plurality of prisms are formed on at least one side of the sheet in a manner that generatrices forming the prisms are in parallel each other so that an incident light entering the light control sheet through the prism-formation side of the light control sheet emerges out of the other side thereof, and it relates also to a surface light source unit and a liquid crystal display device both employing the light control sheet.
Next, an example of a conventional light control sheet will be explained as follows, referring to drawings. FIG. 19 is a diagram showing how a liquid crystal display device employing a conventional light control sheet is structured, and FIG. 20 is an enlarged section of the light control sheet shown in FIG. 19.
In the drawing, the numeral 1 represents a cold-cathode tube, and 2 represents a light-guiding plate wherein prism projection 2b is formed on transmission surface 2a so that light emitted from the cold-cathode tube may emerge obliquely to light control sheet 3.
The light control sheet 3 makes light emerged from the transmission surface 2a to be emerged almost perpendicular to the surface of the light control sheet, to illuminate liquid crystal display element 4.
With regard to the light control sheet 3, in this case, a plurality of prisms of the same shape 3a are formed on surface (incident surface) 3d which faces the light-guiding plate 2 in a manner that generatrices are in parallel each other as shown in FIG. 20. Further, a shape on the section of each prism 3a viewed in the direction that is in parallel with its generatrix is an equilateral triangle.
Namely, in prism 3a, a length of boundary surface at light source side 3b through which light coming from light-guiding plate 2 enters is the same as a length of boundary surface on the side opposite to a light source 3c totally reflecting light toward liquid crystal display element 4.
In a liquid crystal display device structured in the aforesaid manner, light emitted from light source 1 enters light-guiding plate 2, then, is guided through the light-guiding plate 2, and emerges out of the light-guiding plate through its transmission surface 2a. Then, the light enters through boundary surface at the light source side 3b of prism 3a on light-converging sheet 3, and a part of the light directly emerges out of emergent surface 3e and the other part thereof is reflected on boundary surface on the side opposite to a light source 3c and both of them emerge out of emergent surface 3e, to illuminate liquid crystal display element 4.
FIG. 22 shows an example of light distribution on emergent surface 3e indicated when light having characteristics shown in FIG. 21 enters an incident surface of the light control sheet 3. Incidentally, specifications of the light control sheet 3 used for the present conventional example are as follows.
Material . . . Polycarbonate (refractive index n=1.59) PA1 Vertical angle (.theta.1) of boundary surface at light source side 3b of prism 3a=34.degree. PA1 Vertical angle (.theta.2) of boundary surface on the side opposite to a light source 3c of prism 3a=34.degree.
Both boundary surface at light source side 3b and boundary surface on the side opposite to a light source 3c are shown on the expression wherein coordinate axes are established so that an origin of the coordinate passes through a vertex of the prism 3a and generatrices of the prism 3a are perpendicular to X-Y plane, as shown in FIG. 20.
Boundary surface at light source side 3b . . . Y=-1.540X (-0.649 mm.ltoreq.X.ltoreq.0 mm)
Boundary surface on the side opposite to a light source 3c . . . Y=1.540X (0.ltoreq.X.ltoreq.0.649 mm)
On the light control sheet 3 of the present conventional example, the maximum luminance was 2278 (cd/m.sup.2) and a half-value degree (angular range having the luminance that is more than a half of the maximum luminance) was 25.63.degree..
An optical system of the light control sheet 3 mentioned above will be explained as follows, referring to FIGS. 23(a) and 23(b). FIG. 23(a) is a diagram explaining an optical system wherein light shown in FIG. 21 enters light control sheet 3 shown in FIG. 20, while FIG. 23(b) is a diagram on which the optical system shown in FIG. 23(a) is developed.
FIG. 23(b) shows an optical system wherein light enters a slanted transparent plate and then emerges from it. In the case of this type of optical system, when an inclination angle is changed, an angle of emergence from emergent surface 3e only changes. Namely, a graphical shape of light distribution characteristics as shown in FIG. 22 is only subjected to parallel displacement in the direction of an axis of the angle of emergence, and the shape itself can not be controlled.
On the other hand, FIG. 23(a) is equivalent to FIG. 23(b) wherein light enters a slanted transparent plate and emerges from it. Therefore, even when vertical angles (.theta.1, .theta.2) of prism 3a corresponding to a slanted angle are changed, a graphical shape itself can not be controlled though an angle of emergence only changes. Namely, neither the maximum luminance nor the half-value degree can be controlled, and further, side lobe light (undesired light that usually does not enter an eye of a liquid crystal observer) can not be controlled either, which is a problem.
When light emerging from light control sheet 3 is highly directive under the condition that an amount of light emerging from light control sheet 3 is constant independently of a distance from a light source, especially when the emergence luminance in the +10.degree. direction falls rapidly under the condition that light which emerges in the direction to the front (0.degree. direction) is intense, there is a possibility that a liquid crystal observer observes an occurrence of luminance unevenness.
For example, when a liquid crystal image plane in A4 size is observed from the point which is away from the image plane by about 500 mm and is perpendicular to the image plane at its center, an edge of the image plane is inclined by about 10.degree..
FIG. 24 is a diagram showing light distribution characteristics of a light control sheet and luminance unevenness that is caused by the change of an apparent angle to a liquid crystal observer.
Therefore, it is efficient when a peak of emergence is directed to a liquid crystal observer at various points on an image plane instead of directing the peak of emergence to the 0.degree. direction all together on the entire image plane.